Since fuel economies and the need to protect the environment have become priorities, it has become desirable to produce elastomers with good mechanical properties and as low a hysteresis as possible so that they can be used in the form of rubber compositions usable for the manufacture of various semi-finished products involved in the constitution of tires, such as, for example, underlayers, calendering or sidewall rubbers, or treads, and to obtain tires with improved properties, having in particular reduced rolling resistance.
To achieve such an objective, numerous solutions have been proposed, first of all ones essentially concentrating on the use of elastomers modified by means of agents such as coupling, starring or functionalizing agents, with carbon black as the reinforcing filler, with the aim of obtaining a good interaction between the modified elastomer and the carbon black. For it is generally known that in order to obtain the optimum reinforcement properties imparted by a filler, the latter should be present in the elastomeric matrix in a final form which is both as finely divided as possible and distributed as homogeneously as possible. Now, such conditions may only be obtained insofar as the filler has a very good ability firstly to be incorporated into the matrix during mixing with the elastomer and to disagglomerate, and secondly to be dispersed homogeneously in the elastomer.
It is fully known that carbon black has such abilities, which is generally not true of inorganic fillers. For reasons of mutual attraction, the inorganic filler particles have a tendency to agglomerate together within the elastomeric matrix. These interactions have the harmful consequence of limiting the dispersion of the filler and hence the reinforcing properties to a substantially lower level than that which it would be theoretically possible to achieve if all the (inorganic filler/elastomer) bonds that could be created during the mixing operation were in fact obtained. These interactions also tend to increase the consistency of the rubber compositions in the uncured state and therefore to make them more difficult to work (“processability”) than in the presence of carbon black.
However, interest in rubber compositions reinforced with an inorganic filler was greatly revived with the publication of European Patent Application EP 0 501 227, which discloses a sulfur-vulcanizable diene rubber composition, reinforced with a special precipitated silica (SiO2) of the highly dispersible type, which makes it possible to manufacture a tire or tread with substantially improved rolling resistance, without adversely affecting the other properties, in particular those of grip, endurance and wear resistance.
European Patent Application No. EP 0 810 258 discloses a diene rubber composition reinforced by another special inorganic filler, in this case a specific alumina (Al2O3) of high dispersibility, which also makes it possible to obtain tires or treads having such an excellent compromise of contradictory properties.
Although the use of these specific, highly dispersible silicas or aluminas as reinforcing fillers, whether or not as the major component, has reduced the difficulties of processing the rubber compositions that contain them, they are still more difficult to process than for rubber compositions filled conventionally with carbon black.
In particular, it is necessary to use a coupling agent, also known as a bonding agent, the function of which is to provide the connection between the surface of the inorganic filler particles and the elastomer, while facilitating the dispersion of this inorganic filler within the elastomeric matrix.
The term (inorganic filler/elastomer) “coupling agent” is understood in known manner to mean an agent capable of establishing a sufficient chemical and/or physical connection between the inorganic filler and the diene elastomer. Such a coupling agent, which is at least bifunctional, has, for example, the simplified general formula “Y-G-X,” in which:    Y represents a functional group (“Y” function) which is capable of bonding physically and/or chemically with the inorganic filler, such a bond being able to be established, for example, between a silicon atom of the coupling agent and the surface hydroxyl (OH) groups of the inorganic filler (for example, surface silanols in the case of silica);    X represents a functional group (“X” function) which is capable of bonding physically and/or chemically with the diene elastomer, for example by means of a sulfur atom; and    G represents a divalent group making it possible to link Y and X.
The coupling agents must particularly not be confused with simple agents for covering the inorganic filler which, in known manner, may comprise the function Y which is active with respect to the inorganic filler but are devoid of the function X which is active with respect to the diene elastomer.
Coupling agents, in particular (silica/diene elastomer) coupling agents, have been described in a large number of documents, and the best known such coupling agents are bifunctional alkoxysilanes. Thus, in French patent application No. FR 2 094 859, it was proposed to use a mercaptosilane coupling agent for the manufacture of tire treads. It was quickly shown, and is today well known, that the mercaptosilanes, and in particular γ-mercaptopropyltrimethoxysilane or γ-mercaptopropyltriethoxysilane, are capable of giving excellent silica/elastomer coupling properties, but that these coupling agents cannot be used industrially because of the high reactivity of the —SH functions, which very rapidly results in “scorching” (which is premature vulcanization during the preparation of the rubber composition in an internal mixer), in very high Mooney plasticity values and, finally, in rubber compositions which are virtually impossible to work and process industrially. To illustrate this impossibility of using such coupling agents and the rubber compositions that contain them industrially, French patent application no. FR 2 206 330 and U.S. Pat. No. 4,002,594 may be cited.
To overcome this drawback, it has been proposed to replace these mercaptosilane coupling agents by polysulfurized alkoxysilanes, in particular bis-(C1-C4)alkoxysilylpropyl polysulfides such as those described in numerous patents or patent applications. (See, for example, French patent application no. FR 2 206 330 and U.S. Pat. Nos. 3,842,111; 3,873,489, 3,978,103, and 3,997,581.)
These polysulfurized alkoxysilanes are now generally considered to be the products which, for vulcanized rubber compositions filled with silica, give the best compromise in terms of resistance to scorching, processability and reinforcing power. Among these polysulfides, mention must be made of bis-3-triethoxysilylpropyl tetrasulfide (abbreviated to TESPT), which is the (inorganic filler/diene elastomer) coupling agent known to be the most effective, and hence the most used today, in rubber compositions for tires, in particular those intended to form treads for these tires. TESPT is sold, for example, by Degussa under the name “Si69”. However, this product has the known disadvantage that it is very costly and needs to be used most frequently in a relatively large quantity. (See, for example, U.S. Pat. Nos. 5,652,310; 5,684,171; and 5,684,172.)